clk: bcm: rpi: Add an enum for the firmware clocks

While the firmware allows us to discover the available clocks, we need to
discriminate those clocks to only register the ones meaningful to Linux.
The firmware also doesn't provide a clock name, so having a list of the ID
will help us to give clocks a proper name later on.

Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>